As a means for controlling the output (brightness or luminous flux) of a light source by using an alternating current (AC) power source, a phase control system including a semiconductor switching element has been commonly used. Generally, in the phase control system, the switching element is serially connected between the AC power source and the light source (lighting load). The dimming of the light source is performed by controlling the conduction angle within one cycle of the AC voltage (this angle corresponds to the timing at which the switching element is turned on) so as to vary the effective voltage supplied from the AC power source to the lighting load. The “effective voltage” is the square root of the time average of the square of the AC voltage over one cycle. In the case where the AC voltage sinusoidally changes, the effective voltage is equal to 2−0.5 times the maximum value V0 of the AC voltage if no on/off switching task is performed (i.e. if the switch is always on), and its value becomes smaller than 2−0.5 V0 if the on/off switching task is performed.
In one type of conventional dimmer using the phase control system, a zero-crossing point (the point in time at which the AC voltage becomes zero) is used as the reference point for controlling the conduction angle. This type of dimmer includes a zero-crossing detector for detecting the zero-crossing point.
However, the zero-crossing detector may possibly malfunction due to noise contamination or waveform distortion of the AC voltage supplied from the power source. In particular, if the AC voltage is supplied from a power generation by natural energy, such as wind power generation, solar power generation or other techniques which have been drawing attentions in recent years, the power is unstable and it is difficult to completely eliminate the noise or waveform distortion even if the power is controlled by the “smart grid”, i.e. a power grid system having the function of autonomously controlling the electric power supply and demand by means of telecommunication devices and computers. Using an in-house power generation system is also more likely to cause noise contamination or waveform distortion than using the commercial power supply. In the case of a system including a plurality of light sources whose outputs (or brightnesses) need to be individually controlled, the switching circuit for one light source causes a noise or waveform distortion, which may possibly cause a malfunction of the switching element for another light source. Such noise or waveform distortion leads to a brightness fluctuation, flicker or similar problem occurring in the lighting apparatus. These problems are particularly noticeable in the case of an apparatus using an LED as the light source.
One method for more accurately detecting the zero-crossing point uses a phase lock loop (PLL) circuit (for example, see Patent Literature 1). In this system, the zero-crossing detector generates a pulse signal every time it detects the zero-crossing point. The PLL circuit, which includes an oscillator for generating an oscillating signal, receives the pulse signal and outputs the oscillating signal while performing a feedback control to synchronize the oscillating signal with the pulsed signal (i.e. to make these signals in phase with each other). Pulse signals originating from noise or other factors are out of phase with the oscillating signal and hence can be removed. However, the PLL circuit has the problem that, if the response speed is set at a low level, a considerable length of time will be needed to establish or restore the synchronization when the lighting apparatus is energized or the synchronization is broken for some reasons, during which time the amount of light of the apparatus will fluctuate. On the other hand, setting a higher response speed to avoid this problem will lead to an insufficient removal of the noise.
Patent Literature 2 discloses a dimmer in which a fast Fourier transform of the signal of the AC voltage before being sent to the zero-crossing detector is performed to extract only the fundamental component of the AC signal, and this signal, which is free from noise, is sent to the zero-crossing detector so that the zero-crossing point can be detected on the basis of the noise-free signal. However, this dimmer is expensive since it requires a high-performance computing unit to handle a large number of sampled data of the AC voltage signal during the fast Fourier transform operation.